In an analog input device that makes multipoint input of analog signals, isolating transformers are used for insulating the individual points of the analog signals from a ground as shown in FIG. 8 to collect the analog signals at low cost while removing the effect of the ground of the signal sources.
In the analog input device, to input the low frequency analog signals including DC components while insulating from the ground, FET 11 and FET 12, which are a semiconductor switch, interrupt the analog signals successively, thereby producing pulse-like signals with amplitudes corresponding to the analog signals from the secondary sides of the isolating transformers T11 and T12.
In the analog input device, the pulse-like signals produced from the secondary sides of the isolating transformers T11 and T12 are amplified through amplifiers to a prescribed voltage, and one of the pulse-like signals is selected by a switching device SW and is brought into a sample-and-hold circuit.
The sample-and-hold circuit of the analog input device holds an instantaneous amplitude value of the pulse-like signal, and an AD converter converts the instantaneous amplitude value to a digital signal.
Here, FIG. 9 is a diagram for explaining operation timing of the conventional analog input device.
FIG. 9 illustrates that chopping operation is carried out by the FET 11 and FET 12 which turn on and off in response to input pulses to the gates of the FET 11 and FET 12.
The chopping operation causes the output sides (secondary sides) of the transformers T11 and T12 to produce pulse waveforms having amplitude Vt11 and Vt12 and transient responses.
The conventional analog input device is configured in such a manner as to start the AD conversion by sampling and holding the pulse waveform.
For example, the following Patent Document 1 discloses a switch structure of FETs inserted into the primary sides of isolating transformers of a variety of analog signals. It also discloses a configuration for carrying out AD conversion by holding an instantaneous amplitude value with a sample-and-hold circuit.
In addition, the following Patent Document 2 proposes a method of carrying out AD conversion by holding an instantaneous value of an analog signal with a sample-and-hold circuit while a digital circuit stops temporarily to avoid the effect of noise produced from the digital circuit.
Patent Document 1: Japanese Patent Laid-Open No. 58-24232/1983 (FIG. 2).
Patent Document 2: Japanese Patent Laid-Open No. 2006-80646 (Paragraphs [0037] to [0038], and FIG. 3)
With the foregoing configuration, the conventional analog input device holds the instantaneous amplitude value of the pulse-like signal with the amplitude corresponding to the analog signal with the sample-and-hold circuit. However, when receiving induction of external large noise, the amplitude value held by the sample-and-hold circuit fluctuates, offering a problem of bringing about a phenomenon that the AD conversion data is unstable.
Incidentally, in Patent Document 2, the digital circuit is stopped temporarily to avoid the effect of noise produced from the digital circuit. In this case, although the technique is effective for a noise source that can be stopped temporarily, it cannot exclude influences of external noise that cannot be stopped temporarily such as switching noise of a power source.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide an analog input device capable of outputting a high precision digital signal even if it receives induction of large noise from an external noise source that cannot be stopped temporarily.